In electronic equipment for measuring low dc currents, stability and noise often are the limiting factors. One application of such an equipment is found in the medical and chemical analytical instruments, where a small amount of certain gas in a solution, such as oxygen in blood, is to be determined by measuring the current when a polarizing voltage is impressed. In such an application, the dc current level is usually very low, in the sub-nanoampere range. The measurement of such a low current usually encounters the problems of drift and noise. In addition, the physical dimension of such an apparatus often must be miniaturized for insertion into medical probes. For miniaturization, it is desirable to employ integrated circuit (IC) technology, for which complementary metal-oxide-semiconductor (CMOS) field effect transistors (MOSFET) are most widely used. For MOSFETs, the 1/f noise is a serious problem.
The stablity problem is mostly due to temperature effect. Any imbalance in the different components in an IC, such as MOSFETs or resistors, can cause current drift.
At low dc currents, the noise and drift can be comparable to the current to be measured. As a result, stable and accurate low current measurement is difficult to achieve.
In another application for measuring resistance, the principle is usually to divide the voltage across the resistance by the current flowing through the resistance. In practice, it is not desirable to derive the loop current from the applied voltage, because the contact resistance of the voltage probe may give rise to undesirable voltage drop. Therefore, the so-called "Kelvin" measurement is often used for this purpose. In the Kelvin measurement four probes are used. The current is caused to flow in the two outer probes, and the voltage is measured from the two inner probes. In this manner, no current flows in the voltage probes to introduce a voltage drop, and the measured voltage is then accurate. However, it is impractical to use this method to measure the resistance of a liquid solution.